1. Field of the Invention
The present invention relates to photolithography and more particularly to proximity correction features used in photolithography masks used to manufacture semiconductor devices.
2. Description of the Related Art
As semiconductor manufacturing technology is quickly pushing towards the limits of optical lithography, the state-of-the-art processes to date have regularly produced ICs with features exhibiting critical dimensions ("CDs") which are below the exposure wavelength (".lambda."). (A "critical dimension" of a circuit is defined as the smallest width of a feature or the smallest space between two features.) For feature patterns that are designed to be smaller than .lambda., it has been recognized that the optical proximity effect (OPE) becomes much more severe, and in fact becomes intolerable for leading edge sub-.lambda. production processes.
Optical proximity effects are a well known characteristic of optical projection exposure tools. More specifically, proximity effects occur when very closely spaced circuit patterns are lithographically transferred to a resist layer on a wafer. The light waves of the closely spaced circuit features interact, thereby distorting the final transferred pattern features. In other words, diffraction causes adjacent features to interact with each other in such a way as to produce pattern dependent variations. The magnitude of the OPE on a given feature depends on the feature's placement on the mask with respect to other features. The closer together features are to one another, the stronger the optical proximity effect between them.
One of the primary problems caused by such proximity effects is an undesirable variation in feature CDs. For any leading edge semiconductor process, achieving tight control over the CDs of the features (i.e., circuit elements and interconnects) is the number one manufacturing goal, since this has a direct impact on wafer sort yield and speed-binning of the final product.
It has been known that the variations in the CDs of circuit features caused by OPE can be reduced by several methods. One such technique involves adjusting the illumination characteristics of the exposure tool. More specifically, by carefully selecting the ratio of the numerical aperture of the illumination condenser ("NAc") to the numerical aperture of the imaging objective lens ("NAo") (this ratio has been referred to as the partial coherence ratio--.sigma.), the degree of OPE can be manipulated to some extent. The partial coherence ratio is defined as: EQU .sigma.=(NAc)/(NAo)
Generally speaking, as .sigma. increases, the illumination coherence decreases. The less coherent the illumination source, the smaller the OPE. In the extreme case, where .sigma. is greater than 0.7, the OPE can be substantially minimized for feature pitches ("FP") ranging from isolated to semi-isolated, as these terms are defined below. As a rule of thumb, for a modern exposure tool with NAo&gt;0.55, feature packing is often expressed in terms of "pitch" for any two adjacent features. It is noted that for the purposes of the following description, as well as the description of the invention, feature "pitch" is subdivided into the following four categories:
a) Dense features: FP&lt;2.lambda., PA1 b) Semi-dense features: 2.lambda..ltoreq.FP&lt;3.lambda., PA1 c) Semi-isolated features: 3.lambda..ltoreq.FP&lt;5.lambda. and PA1 d) Isolated features: FP.gtoreq.5.lambda., PA1 Halftone Period (HTP)=d+s PA1 For d=s, %H=(d/HTP)*100%=50%
where, FP=feature CD+feature-to-feature space.
In addition to using relatively incoherent illumination, such as described above, OPE can also be compensated for by "pre-correcting" the mask features. This family of techniques is generally known as optical proximity correction (OPC) techniques.
For example, in U.S. Pat. No. 5,242,770 (the '770 patent), which is assigned to the assignee of the present application, the method of using scattering bars (SBs) for OPC is described. The '770 patent demonstrates that the SB method is very effective for modifying isolated features so that the features behave as if the features are dense features. In so doing, the depth of focus (DOF) for the isolated features is also improved, thereby significantly increasing process latitude. Scattering bars (also known as intensity leveling bars or assist bars) are correction features (typically non-resolvable by the exposure tool) that are placed next to isolated feature edges on a mask in order to adjust the edge intensity gradients of the isolated edges. Preferably, the adjusted edge intensity gradients of the isolated edges match the edge intensity gradients of the dense feature edges, thereby causing the SB-assisted isolated features to have the nearly the same width as densely nested features.
While "scatter bar" OPC is an effective method for matching isolated and dense features, the standard scattering bar (SSB) carries more than the required "optical weight" when used in conjunction with semi-isolated feature pitches, and results in an overcorrection if used in this context. In U.S. patent application Ser. No. 08/808,587 filed on Feb. 28, 1997 now U.S. Pat. No. 5,821,014, Applicants of the present invention describe a "feature crowding" OPC method that addresses the need to obtain more precise CD correction for semi-isolated features. The method described therein employs SBs having a lighter optical weight, thereby permitting a more accurate treatment of semi-isolated features.
More specifically, referring to FIG. 1, the "feature crowding" SB method of the Ser. No. 08/808,587 application introduces two new classes of scattering bar: a thin SB 12 ("TSB") having a width that is a fraction of the width of an SSB 14, and a halftone SB 16 ("HSB"), also known as a dashed-SB. The HSB has the same width as the SSB but is broken into dashes using a halftone screen.
In a crowded space between features 18, the optical weight is directly proportional to the SB width. Accordingly, by reducing the width of the TSB, it is possible to crowd the TSB into a tighter feature space. However the minimum width of the TSB is limited by the mask manufacturing process. In today's leading-edge mask manufacturing processes, the minimum manufacturable TSB width is approximately 0.24 .mu.m on the reticle (i.e., feature). At 1.times. wafer scale, this is equivalent to a TSB width of 0.06 .mu.m. As such, if optical weights below 0.06 .mu.m are required, the HSB must be employed.
The HSB was developed to circumvent the foregoing mask manufacturing limitation. Since, the HSB can have the same width as the SSB, it is easier to fabricate and inspect the mask. In addition, by adjusting the halftone period of the HSB (as defined below with reference to FIG. 2), the desired optical weight can be obtained. For example, to obtain a 50% optical weight, the halftone period should be 50% (d=s), as shown in FIG. 2.
Referring to FIG. 2, by adjusting the size ratio between d and s, we can vary the %H to obtain the desired optical weight relative to the SSB. This HSB method extends the optical weight below the manufacturable minimum width of a solid scattering bar imposed by the mask manufacturing process. For example, using a 0.1 .mu.m SSB width as reference, a 25% HSB is equivalent to a 0.025 .mu.m wide scattering bar. This is far below the current 0.06 .mu.m TSB that is achievable by today's advanced mask manufacturing processes.
While the "feature crowding" OPC method of the Ser. No. 08/808,587 application is effective for CD control in semi-isolated feature situations, it is physically impossible to insert any appropriately-sized SB between features in the semi-dense range. For this type of feature, "biasing" must be utilized in order to compensate for the OPE.
The conventional feature biasing method requires modifying the main feature CD by adding or subtracting a pre-determined amount to or from the main feature. For example, a +0.02 .mu.m overall bias on a 0.18 .mu.m main feature alters the main feature width to be 0.20 .mu.m. For semi-dense features, a bias correction of this type is a satisfactory OPC technique, as long as the mask writing tool can resolve the required amount of bias.
In order to perform such a +0.02 .mu.m fine feature bias on a 4.times. DUV wafer pattern, a bias of +0.08 .mu.m (i.e., 0.02 .mu.m.times.4) is required on the reticle. In addition, in order to preserve the pattern symmetry and avoid the positional drift of features, half of this bias amount must be applied to each edge of each candidate feature. The required bias amount per side is then +0.01 .mu.m. At 4.times., this is a mere 0.04 .mu.m. To image such a pattern on a raster e-beam mask writing tool would require a 0.04 .mu.m address unit which, while permissible on the latest e-beam machines, nevertheless can require a writing time in excess of 20 hours for a standard six-inch mask. Clearly, such an extended period of time is neither practical nor acceptable from a production view point. A more acceptable production e-beam writing time is on the order of 3 to 6 hours per mask.
Furthermore, using optical laser mask writing tools for such fine feature biasing is not a practical alternative since such tools lack sufficient resolution. Thus, there remains a high cost barrier discouraging the utilization of ultra-fine biasing of features in the semi-dense feature pitch range. Nevertheless, without some form of fine feature biasing for features in the semi-dense feature pitch range, these pitch ranges cannot be permitted in the layout design rules.
As the semiconductor industry begins to employ additional optical resolution enhancement techniques, (such as alternating phase-shift masks), optical lithography is expected to produce minimum feature dimensions near 0.5.lambda.. For such deep sub-.lambda. circuit design rules, semi-dense features (feature pitches between 2.lambda. and 3.lambda.) will become increasingly common. Thus, the semiconductor industry will soon require a practical OPC solution for semi-dense features for leading edge manufacturing processes. It is the object of the present invention to provide a solution to this problem.